General methods for the production of ammonia are described, for example, in Kirk-Othmer (Kirk-Othmer Encyclopedia of Chemical Technology, electronic edition, last updated Feb. 1, 2008, John Wiley and Sons, Inc., chapter “Ammonia”) and Ullmann's Encyclopedia of Industrial Chemistry (Ullmann's Encyclopedia of Industrial Chemistry, electronic edition, last updated Feb. 1, 2008, Wiley & Sons, Inc., chapter “Ammonia”). Ammonia is usually produced from synthesis gas and nitrogen. The carbon monoxide (CO) present in the synthesis gas is usually converted to carbon dioxide (CO2) before the actual synthesis in a carbon monoxide shift-reaction, it being possible for the most part to separate off said carbon dioxide easily from the hydrogen gas. The removal of residual CO2 generally takes place by gas washing with a solvent which has a high CO2 solubility. This is generally followed also by a methanization, in which CO is converted to methane. Following the conversion of hydrogen and nitrogen to ammonia, the product gas stream comprises, inter alia, traces of methane and inert gases, such as argon or helium. These compounds are generally removed by purging. Subsequently, ammonia is generally condensed out of the gas stream or isolated by gas washing with water as solvent in the form of an aqueous ammonia solution. Liquid ammonia obtained through condensation is generally commercially available in a purity of 99.9% by weight (remainder water and inert gases).
Usually, a further purification of liquid ammonia takes place by converting the ammonia to the gaseous state and either separating off other impurities by distillation, or the impurities are removed by bringing the gaseous ammonia into contact with absorbents, or are catalytically decomposed by bringing said ammonia into contact with a catalyst.
JP-A-2002037623 and US-A-20040091413 disclose, for example, methods for removing oxygen, carbon monoxide and water from commercially available ammonia by bringing gaseous ammonia into contact with catalysts containing manganese oxide and subsequently passing the gaseous ammonia over zeolites. The removal of halide ions is not disclosed.
In KR-A-20020078608, ammonia gas is passed over a thermally treated zeolite in order to remove traces of water and oil, which leads to undesired by-products during the subsequent conversion of ammonia to NF3. The removal of halide ions is likewise not mentioned.
The methods described in the prior art for purifying ammonia have the disadvantage that ammonia has to be converted to the gaseous state and then condensed again. This process is energetically unfavorable and the industrial conversion generally requires high investment.
It was therefore an object of the present invention to provide a method with which halide ions can be removed from liquid ammonia without the ammonia having to be converted to the gaseous state. A purification method of this type is particularly advantageous if, during the storage or the transportation of liquid ammonia, impurities are passed into the ammonia and have to be removed prior to using the ammonia as starting material in a synthesis.
GB-A-862,180 discloses the removal of ionic impurities, for example chloride, from nonaqueous solvents, including ammonia. This disclosure teaches the use of insoluble, open-chain, high molecular weight polymers which comprise ion exchanger groups. According to the disclosure, the open-chain architecture of the polymers is said to make the ion exchanger groups bonded to the polymer easier to reach since in conventional ion exchangers in which the ion exchanger groups are embedded in a crosslinked matrix, the ion exchanger groups can only be accessed with difficulty in nonaqueous solvents. One suitable open-chain polymer with ion exchanger groups that is disclosed is dimethylaminoethylcellulose. Modified alpha-cellulose is specified as a particularly suitable open-chain polymer.
GB-A-862,180 teaches that the ionic impurities can be rinsed off from the ion exchanger resin by rinsing with an aqueous or nonaqueous eluent. In GB-A-862,181, however, it is established that special eluents have to be used to detach certain ionic compounds, such as quaternary ammonium groups, from open-chain polymers with cationic ion exchanger groups.